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SECOND MOST HIGHLY EVOLVED ORGANISM?

August 25, 2015

Since retiring to the West Coast, specifically the Sidney area of Vancouver Island, I have been getting to know a whole new slew of bird species.   While I have seen a fair number of common ravens in my lifetime, they are extremely numerous in my neighborhood and I often hear their deep-throated croaks and assorted other interesting ‘noises’.    They seem to be  even more numerous on Sidney Island, a small inhabited island of about 3,000 acres just a half-hour small boat ride from my marina.  Their large presence there, along with a healthy population of turkey vultures in the warmer months, is no mystery to me.  Over the past decade, the islanders living there have been waging a heated battle with a growing population of  black-tailed deer and particularly fallow deer introduced a century ago as a food source for farmers.  In the interest of helping them curb their numbers, my brother-in-law and I spent a late November weekend this past year contributing to the culling of the deer.  For my more sensitive readers, I do not enjoy the act of killing, but I do like not only the taste but also the healthy quality of the meat of these deer.   To cut to the chase, we were successful in shooting two deer and we field-dressed them on the spot in the woods.  This involved removing their various organs and intestines, which we left in the field for the scavengers.

 

Seemingly within an hour or so, the abundant ravens found the offal and began making a variety of interesting calls.  According to avian vocalization experts, ravens can issue no less than 33 different calls, based on sound and context.   The one I hear most in my area is that ‘deep gurgling croak’ that emanates right from the back of the throat.  Apparently, it can be heard from over a mile away and is usually given to get a response from other ravens in the area.  

 

What really interested me was not so much the quick discovery of the new food source by those ravens, but more the fact that they appeared to immediately and enthusiastically pass the word around of its availability.    One might think that any raven encountering the food would consider keeping the food to itself rather than sharing it with other members of the raven community.  But then I considered the possibility that by calling in other ravens to the feast, which was considerably more food than any one raven could eat even over a several-day period, one would expect the other ravens to reciprocate on other occasions.  This would especially make sense if the ravens in question were somehow related to one another genetically.  Passing on one’s genes can not only be achieved by producing lots of young, but also by looking out for other related individuals which bear some of one’s genes.   Alternatively, I could have been simply watching a territorial pair of ravens “yelling” at their own fledged nestlings to come to the dinner table!    Young ravens are sometimes known to stay with their parents over the first winter.

       

In any case, one might imagine my delight when I perused the literature and found a paper published in the first 2015 issue of the journal, Animal Behaviour, entitled “With Whom to Dine?  Ravens’ Response to Food-Associated Calls Depend on Individual Characteristics of the Caller” by Georgina Szipl of the Department of Cognitive Biology at the University of Vienna in Austria and several colleagues.   However, in reading into the subject a little further, I discovered that the basis for calling by ravens over newly found food items is not so simple!

 

Among the birds, vocalizations produced during foraging for food can serve several functions.   For example, chickadees make contact calls to keep the flock together while seeking food.  Foraging scrub-jays, on the other hand, have sentinel birds, often subordinate individuals, that make alarm calls to warn other genetically related birds about approaching danger.   In ravens, subordinate birds have also been known to make appeasement calls to the more aggressive individuals during interactions at a food source.  Finally, it is also known that vocalizations can be used to indicate the location and quality of the food for a variety of  wildlife species.  

 

To complicate matters even further for the ravens I observed, it turns out that three different types of food-associated calls have been reported: a long “chii” call given by juvenile ravens, a short “who” call uttered by dominant ravens when landing at feeding sites, and the long “haa” call or yell.  The latter vocalization has received the most attention from scientists because it is usually associated with the discovery of a food source and it can be projected for a very long distance.  Among territorial ravens, males, usually the most dominant birds of all,  seldom yell, but among the vagrant, non-breeding birds, yelling is one of the most common vocalizations.   And it is most associated with hungry birds.    It is often given by subordinate ravens when they see food they cannot access because it is already being monopolized by dangerous predators or perhaps a small group of more dominant ravens.  The idea then is to use the “haa” call to recruit and assemble other subordinate ravens to form a bonded group to help overpower those more dangerous or dominant individuals and gain access to the food.   

 

One of the most hotly debated questions about vocalizations in ravens is whether they can actually discriminate between the callers, i.e. identify them individually, and gain useful information about a food source.  For example, knowing the rank of a caller, i.e. whether it is more dominant bird and thus potentially more aggressive, would certainly be useful.  Presumably, it might also be easier to pilfer a food item here and there among a crowd of less dominant birds.  Similarly, experiments with captive ravens demonstrated that these highly intelligent birds are able to differentiate between ignorant and knowledgeable callers, i.e. how reliable they are at passing along information.   In short, being able to tell individuals apart by their calls can help ravens decide whether to join a feeding ‘crowd’ or not.  As an aside, raven researchers prefer the term ‘crowd’ for groups of ravens because they lack the tight cohesion and consistency of membership seen in typical flocks.

 

According to Bernd Heinrich of the University of Vermont and arguably the world’s authority on common ravens, these highly social birds often exhibit a fear-like response initially at arriving at a carcass.  They will land a dozen or so feet away, frequently examine the food, make sudden vertical leaps, peck at it here and there, and then sometimes fly away.   Interestingly, they will often not even approach the food until other birds are in the area.  This is particularly true of juvenile and/or vagrant birds which require having a larger group present to more effectively overwhelm and inhibit aggression by more dominant adults.  Thus, while yelling might be an advantageous behavior for a juvenile or vagrant raven, the dominant adults tend to suppress it, even to the point of attacking the “yellers”.  And fights between ravens are no laughing matter.   They can become extremely vicious, involving pecking, grappling, and biting, and can even prove fatal.  On occasion, a territorial bird might chase an intruder for two or three miles!  Excluding accidents and shooting, one of the main causes of death in common ravens are injuries caused by other ravens.  

 

Back to that recently published study by Georgine Szipl and her colleagues.   They performed recorded playback calls to a free-ranging population of over 200 individually marked ravens that regularly scavenge for food at a local zoo in the Northern Austrian Alps.  The playback recordings consisted of a male and a female as well as a familiar versus an unknown individual. 

 

The ravens responded strongest to the calls of familiar females.  This is explained by the fact that females are lower in rank than males and thus, least likely to be aggressive.  Most important, their study was the first to document under natural conditions that common ravens respond to individual characteristics of the ‘haa’ calls and actually choose whom to approach for feeding, i.e. social allies versus dominant birds of their species.  In other words, by responding to the vocalizations of familiar birds and socializing with them, ravens can reduce aggression, the time and energy spent fighting, and risk of injury.  

 

The main lesson I took home from researching and writing this column was how complex behavior can be sometimes among our birds.   After peeling away the layers of the seemingly simple act of ravens vocalizing upon finding a new food source,  I only found myself asking more questions.   And why should I be surprised?   Common ravens may well be the most intelligent birds of all feathered species and certainly rank right up there as one of the most highly evolved organisms in the world, next to humans.  

           

 

 

                       

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